Deoxyribonucleic Acid & Genome

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Deoxyribonucleic Acid & Genome 

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All organisms possess a scheme to transmit the essential information containing the genetic makeup of the species through successive generations. Human cells have 23 pairs of chromosomes, and each pair contains a unique sequence of genetic information. In the human genome, about 6 × 109 nucleotides, or 3 × 109 pairs of nucleotides, associate in the double helix. The specificity of DNA is determined by the base sequence that is stored in complementary form in the double-helical structure. It facilitates correction of sequence errors and provides a mechanistic basis for replication of information during cell division. Each DNA strand provides a template for replication, which is accomplished by the action of DNA-dependent polymerases that unwind the double-helical DNA and copy each single strand with remarkable fidelity.

  Except for gametocytes, all cells contain the duplicate, diploid number of genetic units, one half of which is referred to as the haploid number. The genetic information contained in chromosomes is separated into discrete functional elements known as genes. A gene is a unit of base sequences that (with rare exception) encodes a specific polypeptide sequence. New evidence suggests that small, noncoding RNAs play critical roles in the expression of this essential information. An estimated 30,000 genes constitute the human haploid genome, and they are interspersed among sequence regions that do not code for protein and whose function is as yet unknown. For example, noncoding RNAs (e.g., transfer RNAs [tRNAs], ribosomal RNAs [rRNAs], other small RNAs) are components of enzyme complexes such as the ribosome and spliceosome. The average chromosome contains 3000 to 5000 genes, which range in size from about 1 kilobase (kb) to 2 megabases (Mb).

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Andreoli & Carpenter's Cecil Essentials of Medicine, 9/e, 2/p